Can I detect unique people in a video?
Can I detect unique people in a video?

Yes. However, for some applications commercial systems are not good enough: German source
The concepts you are looking for are:

Optical Flow: for two frames, define for each point how it moved
Object tracking: given a single object in one frame, find it in the next. Optical flow helps with that.
Object detection: look at YOLO / SSD. Small introduction here.